An automatic transmission vehicle provided with a hydraulic clutch is more expensive and provides poorer fuel economy than a manual transmission vehicle. If the automatic transmission mechanism, based on a manual transmission mechanism, is constructed, it would be possible to facilitate the speed changing operation while also lowering the cost and improving the gas mileage. Examined Published Japanese Patent Application 62-12050 discloses an automatic transmission mechanism adopting a dry type clutch and a synchromesh mechanism on the basis of such manual transmission. A hydraulic shift actuator for stroking a shift fork or a shift fork shaft is constructed such that a flow passage change-over valve is connected to the cylinder chamber of the shift actuator to change the direction of stroke of the shift fork by the flow passage change-over valve so that an idle gear to be fixed by a sleeve ring connected to the shift fork is switched to execute the speed change.
However, in this known automatic transmission mechanism, the flow passage change-over valve controls the shift actuator to make it difficult for the shift actuator to control the pressure that acts on the shift fork. With a large difference in the revolutions per minute (i.e., RPM) between the input side and the output side, therefore, the shift fork or the shift fork shaft is stroked or moved by an excessive force during the speed changing action. Thus, a high shift shock may result, especially in the vicinity of a balk point, by a collision between the input side member and the output side member. As the speed changing rate increases to a significant extent, the aforementioned shift shock also increases. Here, the "balk point" refers to the point at which the input side and the output side in the transmission start to be synchronously rotated and at which a synchronous friction occurs between a synchronizer ring and the cone portion of the idle gear.
In view of the foregoing, a need exists for a control system of an automatic transmission equipped with a synchromesh mechanism that is able to reduce the shift shock and shorten the time period required for a shift change.